Electronic musical instrument having portamento property

ABSTRACT

In a electronic musical instrument wherein a portamento is played by supplying the pitch voltage corresponding to a subsequently depressed key to a capacitor holding the pitch voltage corresponding to a previously depressed key, the charge and discharge currents of the capacitor corresponding to the difference between the two pitch voltages are controlled to vary exponentially thus changing exponentially the capacitor terminal voltage. The terminal voltage of the capacitor is applied to drive a voltage controlled oscillator to vary its oscillation frequency. To vary exponentially the terminal voltage of the capacitor, a mutual conductance converter is connected between the capacitor and a keyboard section and the output current from the mutual conductance converter is controlled by a control signal corresponding to the terminal voltage of the capacitor.

This is a continuation of application Ser. No. 807,084, filed June 6,1977 now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to an electronic musical instrument having aportamento property and capable of continuously varying the tone pitchfrom a frequency corresponding to the note of a first key to thatcorresponding to the note of the a second, subsequently operated, key.

A typical prior art electronic musical instrument having the portamentoproperty is disclosed in U.S. Pat. No. 3,866,836 issued June 3, 1975.The basic construction of an electronic musical instrument of this typewill be described with reference to FIG. 1 of the accompanying drawings.When a key of a keyboard section 11 is depressed there are produced avoltage signal KV (hereinafter termed a pitch voltage) corresponding tothe pitch of the note of the operated key, and a pulse signal KS(hereinafter termed of keying signal) having a width corresponding to aperiod of time during which the key is depressed. The pitch voltage KVis applied to act as an oscillator driving signal to a voltagecontrolled type oscillator 17 (hereinafter called VCO) via voltageholding time constant circuit 12 comprising a switching element 13 inthe form of a field effect transistor, a variable resistor 14, acapacitor 15 and a portamento property selection switch 16 connected inparallel with the variable resistor 14 for producing a tone sourcesignal. The tone source signal is applied to a voltage controlled filter18 (hereinafter termed VCF) to form a musical tone by coloring a tone.The tone signal produced by VCF 18 is subjected to the control of amusical tone level that is an envelope in a voltage controlled typevariable gain amplifier 19 (hereinafter called VCA), and the output ofthis VCA is amplified by an amplifier 20 to produce a tone from aloudspeaker 21. The keying signal KS is applied as a driving signal tothe voltage holding time constant circuit 12 which is used to hold thepitch voltage KV snd to impart the portamento property, and to controlvoltage generators 21, 22 and 23 (hereinafter termed CVG). In responseto the keying signal KS generated by key, these control voltagegenerators CVG's generate time-variable control voltage signalscontrolled by a variety of parameters which are set in a parametercontrol voltage generator 24, and these control voltage signals areapplied to VCO 17, VCF 18 and VCA 19 respectively. In the VCO 17, theoscillation frequency is finely varied in accordance with the controlvoltage signal from CVG 21, while in VCF 18, the cut-off frequency isvaried to form a musical tone signal resembling a natural musical tone.The VCA 19 operates to form a musical tone envelope in accordance with acontrol wave signal. During the normal play the selection switch 16 ofthe voltage holding time constant circuit 12 is closed so as to applythe pitch voltage KV generated by a depressed key directly to VCO 17 viathe selection switch 16 and to store the tone voltage KV in capacitor15. The purpose of capacitor 15 is to hold the pitch voltage KV forobtaining a sustained tone after release of the key while the purpose ofthe switching element 13 is to prevent the discharge of the voltage heldby the capacitor 15.

In an electronic musical instrument having the construction describedabove, where it is desired to provide a portamento property, theportamento property selection switch 16 is opened to charge the pitchvoltage KV in capacitor 15 via variable resistor 14 so that the voltageapplied to VCO 17 varies with a time constant determined by the variableresistor 14 and capacitor 15. A pitch voltage KV₁ corresponding to apreviously depressed key (the first key) is stored in the capacitor 15as shown in FIG. 2 and when a new pitch voltage KV₂ corresponding to asubsequently depressed key (the second key) is generated at time t₁, theterminal voltage of the capacitor 15 increases logarithmically as shownby a solid line in FIG. 2 at a speed corresponding to the time constantdetermined by the variable resistor 14 and capacitor 15. As aconsequence, the oscillation frequency of VCO 17 varies continuously asshown by the solid line in FIG. 2, whereby the pitch varies continuouslyfrom the pitch of the first key to that of the second key. When theaudiences hear such musical sound having the portamento property, sincethe pitch frequency of the tone source signal produced by the VCO 17varies rapidly and then slowly as shown by the solid line curve shown inFIG. 2, it varies differently from the actual pitch variation in thenatural portamento shown by dotted lines in FIG. 2 thus giving anunnatural feeling to the audiences.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide an improvedelectronic musical instrument capable of providing a portamento propertywhose pitch frequency varies like a natural sound.

According to this invention, when supplying a pitch voltagecorresponding to a second key to a capacitor holding a pitch voltagecorresponding to a first key, the charging and discharging currents ofthe capacitor are such that the current that charges the capacitoraccording to the difference between the two pitch voltages variesexponentially thereby causing the terminal voltage of the capacitorwhich drives a voltage controlled oscillator to vary exponentially.

For the purpose of exponentially varying the capacitor terminal voltagea mutual conductance converter is connected between the capacitor andthe keyboard section and a detector is provided for detecting a controlsignal corresponding to the capacitor terminal voltage so as to controlthe output voltage of the mutual conductance converter by the controlsignal.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings

FIG. 1 is block diagram showing the basic construction of a prior artelectronic musical instrument having a portamento property;

FIG. 2 is a graph showing the pitch voltage variation produced by thecapacitor shown in FIG. 1 and such variation in the natural portamento;

FIG. 3 is a connection diagram showing one embodiment of the novelelectronic musical instrument of this invention and having a portamentoproperty;

FIG. 4 is a connection diagram showing one example of the currentcontrolled mutual conductance converter and of the current controllingcircuit shown in FIG. 3;

FIG. 5 is a graph showing a collector current base-emitter voltage of atransistor useful to explain the operation of the converter;

FIGS. 6A and 6B are waveforms showing the input voltage to the converterand the terminal voltage of the capacitor shown in FIG. 3 where thepitch voltage of the second key is higher than that of the first key;

FIGS. 7A and 7B are waveforms showing the converter input voltage andthe capacitor terminal voltage where the pitch voltage of the second keyis lower than that of the first key;

FIG. 8 is a connection diagram showing a modified embodiment of thisinvention; and

FIG. 9 is a connection diagram showing another example of the currentcontrolling circuit shown in FIG. 3.

DESCRIPTION OF THE PREFERRRED EMBODIMENTS

A preferred embodiment of the electronic musical instrument of theinvention having a portamento property shown in FIG. 3 comprises acomparator 32 which compares the pitch voltage KV sent from the keyboardsection 11 shown in FIG. 1 with the output voltage of a source followeramplifier 33 having a high input impedance and a low output impedanceand also serving as a buffer circuit (described below) so that thecomparator 32 produces a zero output when the pitch voltage KV coincideswith the output voltage of the source follower amplifier 33. The pitchvoltage KV is applied to the positive input terminal 32_(a) of thecomparator 32, whereas the output voltage V₀ from the buffer circuit 33is applied to the negative terminal (-). The comparator 32 is alsosupplied with source voltages +V_(s) and -V_(s). A resistance voltagedivider 34 for dividing the output voltage of the comparator 32 isconstituted by resistors 34_(a) and 34_(b) which are connected in seriesbetween the output terminal of the comparator 32 and ground potential. Acurrent controlled mutual conductance converter 36 is provided forcontrolling the output voltage from the voltage divider 34 in accordancewith a control current i produced by a current controlling circuit 35.The positive input terminal (+) of the converter 36 is connected to thejunction between resistors 34_(a) and 34_(b) of the voltage divider 34,while the negative input terminal (-) is grounded via a resistor 36_(a).Source voltages +V_(s) and -V_(s) are also applied to the currentcontrolled mutual conductance converter 36. The output terminal of thisconverter 36 is connected to one terminal of a capacitor 37, the otherterminal of which is grounded. The source follower amplifier 33 adaptedto amplify the terminal voltages of capacitor 37 includes a field effecttransistor 33_(a) of a high input impedance having a drain electrodeconnected to the voltage source +V₅, and a source electrode connected tothe voltage source -V_(s) via a load resistor 33_(b). The outputterminal 33_(c) connected to the source electrode is connected to theVCO 17 shown in FIG. 1. The current controlling circuit 35 for producingthe control current i corresponding to the output voltage from thesource follower amplifier 33 supplies the control current i to thecontrol terminal 36_(b) of the current controlled mutual conductanceconverter 36. The current controlling circuit 35 includes a transistor35_(a) having an emitter electrode connected to the output terminal33_(c) of the source follower amplifier 33 via a variable resistor35_(b), a collector electrode connected to the control terminal 36_(b)of the current controlled conductance converter 36 and a base electrodeconnected to the voltage source -V_(s) via a resistor 35_(c) and to theground through a diode 35_(d). The variable resistor 35_(b) varies thecontrol current i for the purpose hereinafter described.

FIG. 4 shows one example of the current control circuit 35 and thecurrent controlled mutual conductance converter 36 described above. Asshown the converter 36 comprises a pair of NPN type transistors 36_(c)and 36_(d) with their emitter electrodes connected together so as toconstitute a differential amplifier. The base electrode of thetransistor 36_(c) is connected to receive the output voltage V_(i) ofthe voltage divider 34 via positive input terminal (+). The baseelectrode of transistor 36_(d) is connected to one terminal of aresistor 36_(a) via the negative input terminal (-). The commonlyconnected emitter electrodes of transistors 36_(c) and 36_(d) areconnected to the collector electrode of a transistor 36_(l) whichconstitutes a current mirror circuit together with a transistor 36_(m).The collector electrode of the transistor 36_(m) is supplied with theoutput current of the current controlling circuit 35, that is thecollector current of the transistor 35_(a) via control terminal 36_(b)which acts as the control current i for the converter 36. Since a fixedbias voltage is applied to the base electrode of transistor 35_(a), thecurrent i varies in accordance with the input to the current controllingcircuit 35, that is the output V₀ of the source follower amplifier 33.Accordingly, a portion of the control current i proportional to theinput V₀ to the current controlling circuit 35 flows through thecollector electrodes of transistors 36_(c) and 36_(d) as the collectorcurrents I_(c1) and I_(c2). The same current as the collector currentI_(c1) of the transistor 36_(c) flows to the collector electrode of atransistor 36_(f) through PNP transistors 36_(e) and 36_(f) and NPNtransistors 36_(g) and 36_(h) which constitute a current mirror.Similarly, the same current as the collector current I_(c2) of thetransistor 36_(d) flows to the collector electrode of a transistor36_(j) through PNP transistors 36_(i) and 36_(j) which constitute acurrent mirror. The output terminal 36_(k) of the converter 36 isconnected to the juncture between the collector electrodes oftransistors 36_(f) and 36_(g). Consequently, the output current Iderived out from the output terminal 36_(k) is expressed by an equationI=I_(c1) -I_(c2). When an input V_(i) is not supplied to the baseelectrode of the transistor 36_(c) via the positive input terminal (+)I_(c1) =i_(c2) =i/2 so that the output current I is zero. The collectorcurrent i/2 at this time represents the operating point of the converter36 and as the input V_(i) is applied the collector current I_(c1) variesabout the operating point and twice of the variation is taken out as theoutput current I of the converter 36.

In this manner, the mutual conductance gm of the converter 36 isdetermined by the collector currents of transistors 36_(c) and 36_(d).More particularly, the relationship between the collector current I_(c1)of transistor 36_(c) and the base-emitter voltage V_(BE) thereofrepresents the forward characteristic of a diode as shown in FIG. 5. Forthis reason, the collector current I_(c1) of transistor 36_(c) isexpressed by an equation

    I.sub.c1 =I.sub.0 (exp HV.sub.BE -1)                       (1)

where I₀ represents the saturation current, and H a constant. Since themutual conductance gm is equal to current I_(c1) differentiated withrespect to the voltage V_(BE), ##EQU1## Since exp HV_(BE>>) 1, themutual conductance gm can be expressed as follows because it issubstantially proportional to the collector current I_(c1) which in turnis proportional to i/2 and because i/2 is proportional to the controlvoltage V₀ of the current controlling circuit;

    gmaI.sub.C1 ai/2>V.sub.0.

Thus the mutual conductance can be variably controlled by the controlvoltage V₀.

The electronic musical instrument having the portamento property andconstructed as above described operates as follows.

Under a condition wherein capacitor 37 is charged to a voltage V andholds voltage V₀ at the output terminal 33_(c), when a pitch voltage KVis impressed upon the input terminal 32_(a) of the comparator 32 itproduces the difference between the pitch voltage KV and the capacitorvoltage V, which is applied to the voltage divider 34. Denoting thepartial voltage produced by the voltage divider 34 by V_(i) and thecontrol current produced by the current controlling circuit 35 by i, theoutput current I produced by the current controlled mutual conductanceconverter 36 is expressed by the following equation

    I=B.V.sub.i.i                                              (3)

where B represents a constant. The control current i produced by thecurrent controlling circuit 35 is expressed by the following equation.##EQU2## where R represents the resistance value of the variableresistor 35_(b), and V₀ =V-ΔV represents the output voltage of thesource follower amplifier 33, V the charged voltage of capacitor 15, ΔVthe gate-source voltage V_(GS) of the transistor 33_(a) and K' and A'constants. By substituting equation 4 into equation 3, the equation ofthe output current I is modified as follows.

    I=B.V.sub.i.(K'V-A')=K"V-A"                                (5)

where K" and A" represent constants. The terminal voltage V of thecapacitor 37 is expressed as follows.

    V=1/cƒIdt                                         (6)

where C represents the capacitance of the capacitor 37.

By differentiating the both sides of equation 6, we obtain ##EQU3##

Substituting equation 5 into equation 7 ##EQU4## where K and A representconstants. By modifying this equation, we obtain ##EQU5##

By solving this differential equation, we obtain ##EQU6## where Frepresents a constant.

As above described, according to this invention, the terminal voltage Vof capacitor 37 is amplified by the source follower amplifier 33 havinga gain G=1, the output voltage V₀ at the output terminal 33_(c) of theamplifier is converted into a control current i having a magnitudecorresponding to the output voltage V₀ by the current controllingcircuit 35 and the control current i is used to control the outputcurrent I of the current controlled mutual conductance converter 36 sothat the charging current (output current I) flowing through thecapacitor 37 varies exponentially, that is when the control current i issmall the output current I varies in a correspondingly small manner butwhen the control current i is large, the output current I varies in acorrespondingly large manner. Consequently, during the portamento play,while the pitch voltage KV₁ corresponding to the first key is being heldby capacitor 37, when the pitch voltage KV₂ corresponding to the secondkey is applied to the positive input terminal 32_(a) of the comparator32 the pitch voltage KV₂ is compared with (KV_(i) -ΔV) by comparator 32and its differential output is applied to the voltage divider 34 thusproducing a partial pulse voltage V_(i2) as shown in FIG. 6A. This pulsevoltage V_(i2) is applied to the positive input terminal of the currentcontrolled mutual conductance converter 36.

At this time, the control current i applied to the control terminal36_(d) of the converter 36 from the current controlling circuit 35 issmall at first but increases gradually thus controlling the outputcurrent I to increase exponentially. Accordingly, the terminal voltage Vof capacitor 37 charged with this output current I varies exponentiallyas shown in FIG. 6B until a steady state is reached at which the voltageV becomes equal to the applied pitch voltage KV₂. Thereafter, since theoutput of the comparator 32 is zero, this voltage is held. Accordingly,the source follower amplifier 33 produces an output voltage V₀ havingthe same waveform as the terminal voltage V of the capacitor 37 at itsoutput terminal 33_(c) which is supplied to the VCO 17 shown in FIG. 1.Thus, the VCO 17 continuously produces a tone source signal having afrequency corresponding to the variation in the voltage applied theretothus manifesting the portamento property. According to this invention,as shown in FIG. 6B, the voltage wave supplied to the VCO 17 closelyapproximates the pitch variation in the natural portamento (see thedotted line characteristics shown in FIG. 2), and the listeners perceivea natural portamento.

If the control current i of the current controlling circuit 35 is variedby adjusting the variable resistor 35_(b), the variation inclination ofthe converter output current I is changed accordingly so that thecharging speed of the capacitor 37 is controlled. Thus, the tempo ofportamento, i.e. the time for continuously changing a tone from one noteto the other is controlled.

Where the relative magnitude of the pitch voltages produced by the firstand second keys is opposite to that described above, the chargeaccumulated in capacitor 37 at the time of operating the first keydischarges through the current controlled mutual conductance converter36 so that the terminal voltage of the capacitor 37 decreases as abovedescribed. More particularly, when the partial voltage shown in FIG. 7Ais impressed upon the positive input terminal of the current controlledmutual conductance converter 36, the terminal voltage of the capacitor37 decreases exponentially as shown in FIG. 7B, thus producing aportamento tone ranging between from a high pitch to a low pitch.

FIG. 8 shows a modified embodiment of the electronic musical instrumenthaving a portamento property in which elements corresponding to thoseshown in FIG. 3 are designated by the same reference characters. Thismodification differs from that shown in FIG. 3 in the following points,that is the control current i is controlled by the output voltage fromthe source follower amplifier 33 and a tempo control voltage. In otherwords, in this embodiment, the tempo of portamento is controlled byvoltage in contrast to the embodiment shown in FIG. 3 where it iscontrolled by current. For this reason, a current controlled mutualconductance converter 40 having the same construction as the converter36 is included in the current controlling circuit 35. The positive inputterminal (+) of the converter 40 is connected to receive a fractionalportion of the output voltage V₀ of the source follower amplifier 33which is produced by a voltage divider 41 comprising resistors 41_(a)and 41_(b), whereas the negative input terminal of the converter 40 isgrounded through a resistor 40_(a). The control terminal 40_(b) of theconverter 40 is connected to receive the output of the collectorelectrode of a transistor 42_(a). The emitter electrode of thistransistor 42_(a) is connected to receive, via a resistor 42_(b), atempo control voltage generated by a potentiometer (not shown) which is,for example, interlocked with a tempo control member mounted on acontrol panel of the electronic musical instrument. The base electrodeof the transistor 42_(a) is connected to the voltage source -V_(s) via aresistor 42_(c) and to ground through a diode 42_(d). Consequently, theoutput current from the current controlling circuit 35 varies inproportion to the tempo control voltage. As can be clearly noted fromthe foregoing description, the output current from the currentcontrolled mutual conductance converter 40 corresponds to both theoutput voltage of the source follower amplifier 33 and the tempo controlvoltage, and this output current is applied to the control terminal36_(b) of the current controlled mutual conductance converter 36 to actas the control current i. With the portamento playing instrumentdescribed above it is possible to adjust the charging and dischargingspeed of the capacitor 37 by adjusting the tempo control voltage,thereby to control the tempo of portamento.

In the foregoing embodiments the differential output between the pitchvoltages of the first and second keys and produced by the comparator 32is applied to the current controlled mutual conductance converter 36 andwhen the terminal voltage of the capacitor 37 charged by the outputcurrent of the converter 36 becomes equal to an applied pitch voltagethe differential output of the comparator 32 becomes zero so that thecharging of the capacitor 37 is terminated. It is, however, alsopossible to interrupt the charging circuit of the capacitor 37 from theconverter 36 when the terminal voltage of the capacitor 37 becomes equalto the applied pitch voltage where the pitch voltage is applied directlyto the converter 36.

The construction of the current controlled mutual conductance converters36 and 40 is not limited to that shown in FIG. 4 but various other typesmay be used. For example, a CA3080 type linear integrated circuit madeby Radio Corporation of America may be used.

The current controlling circuit 35 for producing the control signalapplied to the current controlled mutual conductance circuit may be acircuit as shown in FIG. 9. In FIG. 9, the output voltage V₀ of thesource follower amplifier circuit 33 is applied to the positive inputterminal of an operational amplifier 45 and the output thereof isapplied to the base electrode of a NPN transistor 46. The emitterelectrode of this transistor is connected to the voltage source -V_(s)via a variable resistor 47 and to the negative input terminal of theamplifier 45. The collector electrode of the transistor 46 is connectedto a current mirror circuit (not shown) so that a current correspondingto the collector current in the transistor 46 is applied to the controlterminal 36_(b) of the converter 36. With this arrangement, the voltageto the negative input terminal of the amplifier 45, that is the emittervoltage of the transistor 46 becomes equal to the voltage V₀. Thereforea current determined by (V₀ -V_(s))/R₄₇ flows to the collector electrodeof the transistor 46 where R₄₇ represents a resistance value of theresistor 47.

Although the mutual conductance converter 36 has been shown anddescribed as being controlled by an independent current, controllingcircuit these elements can be combined into a unitary element. Then theconverter may be changed to a voltage controlled type.

It is also to be understood that the mutual conductance, converter 36can be constituted by three terminal active elements, such as a fieldeffect transistor, and a bipolar transistor. In this case, the inputterminal of the converter is supplied with the pitch voltage from thekeyboard section 11 and one of the inputs is connected to receive afeedback voltage of the output voltage corresponding to the terminalvoltage of capacitor 15.

As above described according to the portamento playing instrument ofthis invention the charging current of a capacitor which holds the pitchvoltage at the time of playing a portamento is controlled to be variedexponentially and the terminal voltage of the capacitor is used to drivea voltage controlled type oscillator which serves as the tone sourcecircuit as that the frequency of the tone source signal generated by thevoltage controlled type oscillator varies exponentially thus closelyapproximating the frequency variation in the natural portamento.Accordingly, it is possible to play the natural portamento by anelectronic musical instrument.

It should be understood that the invention is not limited to thespecific embodiments described above and that many changes andmodifications will be obvious to one skilled in the art.

What is claimed is:
 1. In an electronic musical instrument including acapacitor which is charged with a pitch voltage to hold the same, and avoltage controlled type oscillator which is controlled by the terminalvoltage of said capacitor for playing a portamento, the improvementwhich comprises a first mutual conductance converter responsive to aninput voltage signal to produce a current signal for varying saidcapacitor terminal voltage, means for supplying the current signal ofsaid mutual conductance converter to said capacitor, means for detectingsaid capacitor terminal voltage to produce a control voltage signalcorresponding to said capacitor terminal voltage, means coupled to saidmutual conductance converter and said detecting means for controllingthe current signal of said mutual conductance converter in accordancewith said control voltage signal so that said current signal issubstantially directly proportional to said capacitor terminal voltagethereby causing said capacitor terminal voltage to vary exponentiallywith respect to time both in ascending and descending characteristics,and a comparator responsive to said pitch voltage and said capacitorterminal voltage for delivering to said mutual conductance converter anoutput voltage signal as said input voltage signal to said converterwhen said capacitor terminal voltage is in a predetermined relation inmagnitude to said pitch voltage, said voltage controlled type oscillatorhaving a substantially linear input-output characteristic therebyproducing a tone signal having a frequency varying exponentially withrespect to time in said both characteristics.
 2. The electronic musicalinstrument according to claim 1 wherein said mutual conductanceconverter is of a current controlled type.
 3. The electronic musicalinstrument according to claim 2 wherein said mutual conductanceconverter comprises first and second transistors with their emitterelectrodes commonly connected so as to constitute a differentialamplifier, first and second current mirror circuits for deriving out acurrent variation corresponding to the collector currents of saidtransistors, a circuit for deriving out the difference between theoutput currents from said first and second current mirror circuits assaid current signal, and a circuit for applying a control currentcorresponding to said control voltage signal to the emitter electrodesof said first and second transistors.
 4. The electronic musicalinstrument according to claim 4 wherein said current signal controlmeans is connected to supply to said mutual conductance converter acontrol current corresponding to the control voltage signal of saiddetecting means which is proportional to the terminal voltage of saidcapacitor.
 5. The electronic musical instrument according to claim 5wherein said current signal control means comprises a bipolar transistorwith its base electrode supplied with a fixed bias potential, theemitter electrode supplied with the output control voltage signal fromsaid detecting means, and the collector electrode connected to supply acollector current proportional to said control voltage signal to saidmutual conductance converter.
 6. The electronic musical instrumentaccording to claim 5 wherein said current signal control means furthercomprises a variable resistor connected between said detecting means andthe emitter electrode of said bipolar transistor to control a tempo ofthe portamento.
 7. The electronic musical instrument according to claim4 wherein said current signal control means comprises a bipolartransistor and an amplifier, and wherein the control voltage signal fromsaid detecting means is applied to the base electrode of said bipolartransistor via said amplifier, the emitter electrode of said transistoris connected to a source through a load, and the collector electrode isconnected to apply the collector current corresponding to the controlvoltage signal to said mutual conductance converter.
 8. The electronicmusical instrument according to claim 1 wherein said detecting meansincludes a buffer circuit having a high input impedance and a low outputimpedance, the terminal voltage of said capacitor is applied to theinput of said buffer circuit and the output from said buffer circuit, isapplied to said current signal control means.
 9. The electronic musicalinstrument according to claim 8 wherein said buffer circuit comprises asource follower amplifier including a field effect transistor.
 10. Theelectronic musical instrument according to claim 1 which furthercomprises a voltage dividing circuit connected between said comparatorand said mutual conductance converter.
 11. The electronic musicalinstrument according to claim 1 wherein said current signal controlmeans comprises a second mutual conductance converter connected toreceive the control voltage signal from said detecting means, a currentcontrolling circuit for supplying to said second mutual conductanceconverter a control signal corresponding to a portamento tempo controlvoltage, and means for applying to said first mutual conductanceconverter the output from said second mutual conductance converter toact as the control signal for the first mutual conductance converterthereby controlling the output current signal of the first mutualconductance converter in accordance with said tempo control voltage..Iadd.
 12. In an electronic musical instrument including a capacitorwhich is charged with a pitch voltage signal to hold the same, and avoltage controlled type oscillator which is controlled by the terminalvoltage of said capacitor for playing a portamento, the improvementwhich comprises means responsive to said pitch voltage signal forgenerating a current signal to vary said capacitor terminal voltageexponentially through charging and discharging said capacitor inaccordance with said current signal, means for detecting said capacitorterminal voltage to produce a control signal, means coupled to saidcurrent signal generating means for controlling the same in response tosaid control signal so that said current signal increases and decreasesin magnitude when said capacitor terminal voltage increases anddecreases exponentially, respectively, and means for causing saidcurrent signal generating means to cease from generating said currentwhen said capacitor terminal voltage is in a predetermined relation inmagnitude to said pitch voltage signal. .Iaddend.